Oxidative damage of proteins has been implicated in disease and aging. In vitro studies demonstrate that two unnatural amino acids, o,o4-dityrosine and o-tyrosine, are stable markers of protein oxidation. We have investigated the possibility that assaying these compounds in urine could provide a noninvasive way to determine levels of protein oxidation in vivo. Isotope dilution gas chromatography/mass spectrometry was used to quantify levels of o,o4-dityrosine and o-tyrosine in skeletal muscle and urine of aging rats subjected to two interventions > dietary antioxidant supplementation and exercise training. In both sedentary rats and exercise-trained rats, antioxidant therapy reduced levels of protein-bound o,o4-dityrosine in skeletal muscle. In contrast, antioxidant therapy or exercise training minimally affected o-tyrosine levels in this tissue. Levels of the oxidized amino acids in urine samples mirrored those of skeletal muscle proteins. Quantification of the lev els of oxi dized amino acids in urine may thus serve as a noninvasive measure of oxidative stress in vivo because they change in parallel with levels of protein-bound oxidized amino acids in skeletal muscle.